If you have want to prevent influenza and have severely low vitamin D
the number needed to prevent with Vitamin D is 4
the number needed to prevent with flu shots is 40
- “Most recently, a scientific review [5,6] of 25 randomized controlled trials confirmed that vitamin D supplementation boosts immunity and cuts rates of cold and flu.”
- “"… [P]eople who took daily or weekly vitamin D supplements were less likely to report acute respiratory infections, like influenza or the common cold, than those who did not …”
- “For people with the most significant vitamin D deficiencies (blood levels below 10 [ng/mL]), taking a supplement cut their risk of respiratory infection in half.
- ”People with higher vitamin D levels also saw a small reduction in risk: about 10 percent, which is about equal to the protective effect of the injectable flu vaccine, the researchers say.”
- “Overall, one person would be spared from influenza for every 33 people taking a vitamin D supplement (NNT = 33), whereas 40 people have to receive the flu vaccine in order to prevent one case of the flu (NNT = 40).”
- “Among those with severe vitamin D deficiency at baseline, the NNT was 4. In other words, if you're vitamin D deficient to begin with, vitamin D supplementation is 10 times more effective than the flu vaccine.”
- “A large-scale, systematic review  of 51 studies, published in the Cochrane Database of Systematic Reviews in 2006, found no evidence that the flu vaccine is any more effective than a placebo in children under two. The studies involved 260,000 children, age 6 to 23 months.
- “In a 2012 review, Cochrane concluded that "in children aged from two years, nasal spray vaccines made from weakened influenza viruses were better at preventing illness caused by the influenza virus than injected vaccines made from the killed virus. Neither type was particularly good at preventing "flu-like illness" caused by other types of viruses. In children under the age of two, the efficacy of inactivated vaccine was similar to placebo.”
- “Compared to children who do not get an annual flu vaccine, those who receive influenza vaccinations have a three times higher risk of hospitalization due to influenza. ”